The present invention relates to an air-fuel ratio diagnostic device for an internal combustion engine that includes an air-fuel ratio sensor, which detects oxygen concentration in exhaust at a junction of an exhaust manifold where the exhaust flowing from a plurality of cylinders meet, and determines variation between the cylinders based on the detection of the air-fuel ratio sensor.
Japanese Patent Application Laid-Open No. 2003-138962 describes an example of the above-described air-fuel ratio diagnostic device for an internal combustion engine. Based on detection data from an air-fuel ratio sensor, the air-fuel ratio diagnostic device calculates a variation amount, which is the difference between the two peak values in the detection data from the air-fuel ratio sensor, to determine whether there is a variation between cylinders.
However, in the structure that determines a variation between cylinders based on only the variation amount of the air-fuel ratio, for example, when a target air-fuel ratio AFX1 is altered to a leaner target air-fuel ratio AFX2, the problem described below may occur.
When the air-fuel ratio changes toward the target air-fuel ratio AFX2 due to the alteration of the target air-fuel ratio, the variation amount, which is calculated from the difference between the two peaks, may exceed a determination value even when there is actually no variation between cylinders. In this case, based on the variation amount that has exceeded the determination value, a determination is made that variation is occurring between cylinders. Thus, the determination does not accurately reflect the actual air-fuel ratio.
In this manner, in the example of the air-fuel ratio diagnostic device disclosed in Japanese Patent Application Laid-Open No. 2003-138962, a problem may occur when determining variations between cylinders. Such a problem may also occur for determinations related to variations between cylinders, such as when determining the degree of variation between cylinders and when determining whether the variation between cylinders is resulting in a leaner or richer air-fuel ratio.